[ricm]

I've heard the HPI units work fine but are noisy and require quite a lot of space so you have to move the servos and/ or gearbox location.

GardenTrucking do a 4:1 reduction which bolts directly onto the back of the gearbox which might be a better option - all you need is another (shorter) drive shaft. I didn't take this option because I thought 4:1 would be a bit too slow, but is better for anybody with less confidence with DIY mechanics..